backlight: aw99706: Fix build errors caused by wrong gpio header

backlight: aw99706: Fix build errors caused by wrong gpio header

The driver uses GPIO descriptor API (devm_gpiod_get,
gpiod_set_value_cansleep, GPIOD_OUT_LOW) but includes the legacy
<linux/gpio.h> header instead of <linux/gpio/consumer.h>.

When CONFIG_GPIOLIB is not set, <linux/gpio.h> does not include
<linux/gpio/consumer.h>, causing build errors:

  error: implicit declaration of function 'gpiod_set_value_cansleep'
  error: implicit declaration of function 'devm_gpiod_get'
  error: 'GPIOD_OUT_LOW' undeclared

Fix by including the correct header <linux/gpio/consumer.h>.

Fixes: 147b38a5ad06 ("backlight: aw99706: Add support for Awinic AW99706 backlight")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512171631.uKXlYwqu-lkp@intel.com/
Signed-off-by: Junjie Cao <junjie.cao@intel.com>
Reviewed-by: Daniel Thompson (RISCstar) <danielt@kernel.org>
Link: https://patch.msgid.link/20260111130117.5041-1-junjie.cao@intel.com
Signed-off-by: Lee Jones <lee@kernel.org>